Anti-skid control apparatuses for executing an ABS control by controlling a brake fluid pressure (hereinafter referred to as a wheel cylinder pressure) within a wheel cylinder have been mounted on a wide variety of vehicles. Such anti-skid control apparatus generally includes a normally opened solenoid valve (pressure-increasing valve) and a normally closed solenoid valve (pressure-reducing valve). The normally opened solenoid valve is provided at a hydraulic pressure circuit connecting the master cylinder to the wheel cylinder and the normally closed solenoid valve is provided at a hydraulic pressure circuit connecting the wheel cylinder to a reservoir. The master cylinder generates a brake fluid pressure (hereinafter, referred to as a master cylinder pressure) responsive to a brake operation by a driver. In this configuration, a pressure-reducing control, a sustaining control and a pressure-increasing control for the wheel cylinder pressure are executed by controlling the pressure-increasing valve and the pressure-reducing valve respectively.
The ABS control is generally started and executed for the vehicle wheel at which a predetermined ABS control start condition is established. Specifically, the ABS control is achieved by executing the pressure-increasing control at least after the pressure-reducing control is executed. When the ABS control start condition is established again while the pressure-increasing control in the current ABS control is being executed, the pressure-increasing control is terminated and a next ABS control (a pressure-reducing control of a next ABS control) is started continuously. That is, considering that a period of time from where the ABS control start condition is established to where the ABS control start condition is established in the next place is referred to as a control cycle, generally, an ABS control is carried on with multiple and continuous control cycles.
Recent requirements have led to a control for smoothly (steplessly) increasing the wheel cylinder pressure during the pressure-increasing control. Hereinafter, such control is referred to as a linear pressure-increasing control. In order to cope with this situation, as disclosed in JP2003-19952A, a linear solenoid valve, especially a normally opened linear solenoid valve, has been adopted as a pressure-increasing valve for the anti-skid control apparatus. Specifically, such linear solenoid valve can (steplessly) modify a pressure difference between the master cylinder pressure and the wheel cylinder pressure in accordance with a value of electric current value linearly controlled.
As for a normally opened linear solenoid valve, generally, a pressure difference, which corresponds to a suction force, varies in proportion to a value of electric current (command electric current). The pressure difference is hereinafter referred to as a command pressure difference. Accordingly, the normally opened linear solenoid valve serving as the pressure-increasing valve is controlled so as to disconnect a fluid communication between the master cylinder and the wheel cylinder when the command pressure difference determined in accordance with the electric current value is larger than an actual pressure difference. The normally opened linear solenoid valve is controlled so as to establish the fluid communication therebetween when the command pressure difference is smaller than the actual pressure difference. As a result, the brake fluid flows from the master cylinder-side into the wheel cylinder, and an actual pressure difference becomes small as the wheel cylinder pressure is increased. When the actual pressure difference becomes equal to the command pressure difference, the actual pressure difference matches the command pressure difference.
In other words, in order to achieve the linear pressure-increasing control with a normally opened linear solenoid valve as a pressure-increasing valve, with the pressure-reducing valve in a closed state, first of all, a value of actual electric current for the normally opened linear solenoid valve is set to a value of electric current corresponding to an actual pressure difference at a starting point of the linear pressure-increasing control. The value of electric current corresponding to the actual pressure difference is a value of electric current for matching the command pressure difference with the actual pressure difference and is hereinafter referred to as an actual pressure difference corresponding electric current value. Subsequently, the value of actual electric current is needed to be linear reduced with a constant gradient corresponding to a target gradient for increasing the wheel cylinder pressure. Thus, from the starting point of the linear pressure-increasing control, the actual pressure difference is smoothly reduced, as a result, during the linear pressure-increasing control, the wheel cylinder pressure can be smoothly increased having a gradient that is identical to the target increasing gradient.
When the pressure reducing control or the pressure-sustaining control is shifted to the linear pressure-increasing control, the linear solenoid valve travels from a closed state to an opened state. In here, for example due to a delay in opening the linear solenoid valve that may occur as a result of a response delay thereof (control delay), there is a possible case that the wheel cylinder pressure underruns a value corresponding to the basic electric current value (target value) at an early stage of the linear pressure-increasing control. Hereinafter, such delay is referred to as a wheel cylinder pressure-increasing delay. The wheel cylinder pressure-increasing delay becomes remarkable as a gradient of the basic electric current value becomes larger or increases, i.e., as the target gradient for increasing the wheel cylinder pressure becomes larger or increases.
In the light of the forgoing, JP2005-30453A discloses therein that a value of actual electric current of the linear solenoid valve is intentionally shifted from the basic electric current value by a predetermined electric current amount (first electric current amount) in a direction to reduce the actual pressure difference. Here, “a direction to reduce the actual pressure difference” represents a direction for reducing a value of actual electric current for the normally opened linear solenoid valve and a direction for increasing a value of actual electric current for the normally closed linear solenoid valve. As a result, a valve body of the linear solenoid valve travels easily in an opening direction and a delay in opening the valve body is restrained, which restrains a wheel cylinder pressure-increasing delay.
However, in a case where the gradient of the basic electric current value is large, even though a value of actual electric current of a linear solenoid valve is changed from the basic electric current value during the linear pressure-increasing control by the first electric current amount in a direction for reducing the actual pressure difference, the inventors verified that the wheel cylinder pressure-increasing delay becomes large.
The present invention has been made in view of the above circumstances and provides an anti-skid control apparatus which executes an ABS control with a linear solenoid valve as a pressure-increasing valve and effectively restrains a wheel cylinder pressure-increasing delay.